Gas component detecting element and detector

ABSTRACT

Disclosed is a method for producing a gas component detecting element for detectors which comprises a sintered body of particles of a metal oxide wich changes in electrical resistance depending on relative atmosphere of combustible components in an exhaust gas, catalyst particles being supported on the surface of particles of the metal oxide, the method comprising the following steps: 
     preparing particles of the metal oxide, 
     calcining the particles of the metal oxide, 
     subjecting the calcined particles to a treatment for enlarging specific surface area of the calcined particles, 
     supporting catalyst particles on the surface of the particles subjected to the enlargement treatment, and 
     sintering the particles which support the catalyst particles. 
     The detector which has the above-mentioned gas component detecting element is also disclosed.

BACKGROUND OF THE INVENTION

The present invention relates to a method for producing a gas componentdetecting element and a detector.

Hitherto, it has been known to use a gas component detecting elementcomprising titanium dioxide for indirect detection of air-fuel ratio ofinternal-combustion engine. This known method utilizes the property oftitanium dioxide to change its electrical resistance depending onconcentration of the gas component. In this way the concentration of thegas component is detected by the change of electrical resistance, andair-fuel ratio of internal-combustion engine is indirectly detectedbased on the detected concentration of gas component.

In the case of this gas component detecting element comprising titaniumdioxide, titanium dioxide alone is slow in absorption and resorption ofgas component and so low in sensitivity and therefore, conventionally aplatinum/rhodium catalyst was supported on the surface of titaniumdioxide particles for enhancement of sensitivity (e.g., Japanese PatentKokai No. 56-112638).

Internal-combustion engines are used at temperatures in a very widerange (200°-1000° C.). Therefore, particles of the platinum/rhodiumcatalyst easily move about on the surface of titanium dioxide particlesand they gather and agglomerate to result in growth of particles dueespecially to the heat under high temperature.

As a result, catalytically active areas on the surface of titaniumdioxide particles decrease and absorption and resorption of gascomponent on the surface of titanium dioxide particles become slow tocause reduction of sensitivity for gas component.

The inventors have conducted intensive research on these points and havefound the cause for movement of catalyst particles under hightemperature. That is, it is postulated that catalyst particles firmlybond to surface defects (called active points) of titanium dioxideparticles with bonding of S.M.S.I. (Strong Metal Support Interaction) asdisclosed, for example, in "American Chemical Society", pages 2870-2874published in U.S.A. in 1979. The surface defect of titanium dioxidemeans lattice defect caused by escaping, from the crystal lattice, ofunpaired electrons (dangling bond), oxygen or the like on the surface oftitanium dioxide. Bonding to such surface defects result in strongadherence of the surface of titanium dioxide particles and catalystparticles.

However, the surface defects (active points) of titanium dioxideparticles disappear owing to reduction of specific surface areaaccompanied by growth of particles, and bonding of defects to each otherand rearrangement of defects by the heat at heat treatment, namely,calcination which is a pretreatment for stably obtaining the finaldesired product, namely, titanium dioxide sintered body (for reducingheat shrink and obtaining desired density). Thus, the surface defectdensity markedly decreases.

As a result, even if catalyst particles are supported on titaniumdioxide particles having a low surface defect density, a proportion ofthese catalyst particles which bond to the surface defects is very smalland thus movement of catalyst particles is brought about due to heat.

The present invention aims at inhibition of particle growth bypreventing movement of catalyst particles.

SUMMARY OF THE INVENTION

The present invention has been accomplished by the research conducted bythe inventors in view of the above problems and includes the technicalmeans of calcining titanium dioxide particles, increasing the amount ofsurface defects on the surface of the calcined titanium dioxide than theamount of surface defects before calcination, supporting a catalyst onthe surface of titanium dioxide subjected to the above treatment, andsintering the titanium dioxide particles having the catalyst supportedthereon.

That is, the present invention provides the following method forproducing a gas component detecting element and the following detector.

(1) A method for producing a gas component detecting element fordetectors which comprises a sintered body of particles of a metal oxidewhich changes in its electrical resistance depending on relativeatmosphere of combustible components in an exhaust gas, catalystparticles being supported on the surface of particles of the metaloxide, the method comprising the following steps:

preparing particles of the metal oxide,

calcining particles of the metal oxide,

subjecting the calcined particles to a treatment for enlarging specificsurface area of the calcined particles,

supporting catalyst particles on the surface of the particles subjectedto the enlargement treatment, and

sintering the particles on which the catalyst particles are supported.

(2) A method for producing a gas component detecting element fordetectors which comprises a sintered body of particles of titaniumdioxide which change in its electrical resistance depending on relativeatmosphere of combustible components in an exhaust gas, catalystparticles being supported on the surface of the particles of titaniumdioxide, the method comprising the following steps:

preparing particles of titanium dioxide,

calcining the particles of titanium dioxide,

subjecting the calcined particles of titanium dioxide to a grindingtreatment so that ratio specific surface area (A₁) of the particles oftitanium dioxide at calcination before grinding and specific surfacearea (A₂) of the particles of titanium dioxide after grinding satisfiesthe relation A₁ /A₂ <1.0,

supporting catalyst particles on the surface of the particles oftitanium dioxide subjected to grinding treatment, and

sintering the particles of titanium dioxide on which the catalystparticles are supported.

(3) A method for producing a gas component detecting element fordetectors which comprises a sintered body of particles of titaniumdioxide which changes in its electrical resistance depending on relativeatmosphere of combustible components in an exhaust gas, catalystparticles being supported on the surface of the particles of titaniumdioxide, the method comprising the following steps:

preparing particles of titanium dioxide,

calcining the particles of titanium dioxide,

subjecting the calcined particles of titanium dioxide to a grindingtreatment so that ratio of specific surface area (A₁) of the particlesof titanium dioxide a calcination before grinding and specific surfacearea (A₂) of the particles of titanium dioxide after grinding satisfiesthe relation A₁ /A₂ <1.0,

impregnating the particles of titanium dioxide subjected to grindingtreatment with a mixed solution of chloroplatinic acid and rhodiumchloride,

adding an organic binder to the particles of titanium dioxideimpregnated with the mixed solution to produce a paste,

provided on the surface of a substrate the pasty particles of titaniumdioxide in the form of a film of a given thickness, and

sintering the pasty particles of titanium dioxide provided on thesurface of substrate at 800°-1300° C.

(4) A detector which has a gas component detecting element comprising asintered body of particles of a metal oxide which changes in itselectrical resistance depending on relative atmosphere of combustiblecomponents in an exhaust gas, catalyst particles being supported on theparticles of metal oxide wherein the catalyst particles comprise finecatalyst particles of about 1 nm-about 30 nm and coarse catalystparticles of at least about 100 nm, the fine catalyst particles and thecoarse catalyst particles are supported on the particles of metal oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-12 are explanations of the effect of the present invention aremicrophotographs which show particle structures of particles of titaniumdioxide and catalyst particles.

FIG. 13 and FIG. 14(a) and (b) show a sensor structure of one example ofthe present invention. FIG. 13 is a general cross-sectional view andFIG. 14(a) is an oblique view of gas component detecting element partand FIG. 14(b) is a cross-sectional view of the gas component detectingelement part illustrated in FIG. 14(a), taken along line A--A of FIG.14(a).

FIG. 15 is a schematic view of an exhaust system of engine forexplanation of the present invention.

FIG. 16 show characteristic graphs for explanation of effect of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Method for increasing amount of the surface defects includes, forexample, grinding particles of titanium dioxide or dissolving thesurface of particles of titanium dioxide with inorganic acid or organicacid.

When particles of titanium dioxide are ground, fresh faces are producedat the surfaces of the ground particles. At the fresh faces, unpairedelectrons are formed because the portion at which particles are bondedto each other is forcedly torn off by the grinding. For this reason,amount (density) of surface defects increases.

Ratio of specific surface area of the surface of particles of titaniumdioxide before and after grinding has the relation of (specific surfacearea before grinding/specific surface area after grinding)<1.0. Thisratio means that specific surface area after grinding is larger thanthat before grinding and as a result, amount of surface detects oftitanium dioxide increases.

On the other hand, dissolution of the surface of particles of titaniumdioxide with inorganic acids such as hydrochloric acid, nitric acid,phosphoric acid and hydrofluoric acid or organic acids results in freshfaces on that surface and for the same reason as above, the amount ofsurface defects of titanium dioxide increases.

The metal oxide which changes in its electrical resistance according tothe relative atmosphere of combustible component in exhaust gasincludes, for example, cobalt oxide, tin oxide, nickel oxide, zinc oxideand the like as well as titanium dioxide and any of them can be used inthe present invention.

The catalyst used in the present invention is an oxidation catalyst andit may comprise a platinum/rhodium mixture or this mixture to which isadded at least one metal selected from the group consisting of noblemetals such as rhodium, palladium, and iridium, cobalt, nickel,manganese, iron, copper, technetium, silver, rhenium, osmium, and gold,or may comprise platinum or rhodium alone or a single substance selectedfrom the metal catalysts of the above group.

Among them, platinum/rhodium mixture is preferred. This is becauserhodium is higher than platinum in heat resistance and presence ofrhodium can further inhibit growth of catalyst particles.

In the present invention, the catalyst particles can be supported on thesurface of particles of titanium dioxide by impregnating the surface ofparticles of titanium dioxide with the catalyst in the form of ametallic salt solution and heat treating the particles. The metallicsalt solution includes, for example, nitric acid solution, hydrochloricacid solution, ammonium solution, cyanic acid solution, sulfuric acidsolution, and the like. Furthermore, it is also possible to supportcatalyst particles on the surface of particles of titanium dioxide bymixing an organic solvent with particles of the metal oxide, molding themixture into a bulk of a desired shape, impregnating this bulk with theabove-mentioned catalyst solution, drying the bulk and then heattreating the bulk at a given temperature. Moreover, catalyst particlescan be supported on the surface of particles of titanium dioxide byreduction decomposition process or by photoelectrodeposition process.

Calcination temperature is preferably between 800° C.-about 1300° C.This range of calcination temperature is necessary for obtaining adesired density of titanium dioxide sintered body as a final objectiveproduct and for reducing heat shrinkage of the product. About 800° C. isa lower limit for obtaining rutile crystal and about 1300° C. is anupper limit for being lower than sintering temperature.

In the present invention, it is preferred to carry out a heat treatmentfor stabilization after supporting catalyst particles on the surface ofparticles of titanium dioxide.

Catalyst particles are supported not only on the surface defects on thesurface of particles of titanium dioxide, but also on the surface freefrom surface defects. In this case, the catalyst particles supported onthe surface free from surface defects rapidly grow by temperature andexert a great influence on catalyst activity and so it is necessary tohave saturated the growth of particles to some extent. This saturationtemperature is higher than about 1000° C.

Since this heat treating temperature is included in the range of about100° C.-about 1300° C. which is the range of sintering temperature fortitanium dioxide particles, this heat treatment can be carried outsimultaneously with sintering of particles of titanium dioxide.

Such heat treatment does not cause substantial growth of catalystparticles bonded to surface defects of particles of titanium dioxide. Onthe surface of particles of titanium dioxide are supported fine catalystparticles of about 1 nm-about 30 nm and coarse catalyst particles of atleast about 100 nm which have finished growing.

In the present invention, the titanium dioxide sintered body may notonly be in the form of a thin film referred to hereinafter, but also bein the form of a bulk. Furthermore, particles of titanium dioxide as rawmaterial may also be of anatase-type crystal structure. They aretransformed to rutile-type crystal structure by the heat treatment.

Moreover, molding and sintering of particles of titanium dioxide can besimultaneously carried out by employing hot pressing method.

In the present invention, surface defects of particles of titaniumdioxide can be increased and so catalyst particles which bond to thesurface defects increases.

According to the present invention, growth of catalyst particles can beavoided and as a result, reduction of catalytic activity can beinhibited. Thus, the present invention exhibits excellent practicaleffect that sensitivity for detection of gas components can be stablymaintained.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be explained by the following examples,it being understood that these examples are not intended to limit theinvention thereto.

First, construction of titanium dioxide type oxygen concentration sensorto which the present invention is applied will be explained.

In FIG. 13 and FIG. 14(a) and (b), sensor comprises oxygen detectingpart 1, housing 2 and terminal part 3 as in FIG. 13. Detecting part 1has the construction as shown in FIG. 14(a) and (b) wherein lower levelportion 7a (smaller thickness portion) is formed at the end ofrectangular substrate 7 of aluminum oxide and thin film layer 8 ofaluminum oxide which has therein heater wire 6 made by sinteringplatinum paste is formed on the lower level portion 7a. A pair ofelectrode wires 9 made by sintering platinum paste are formed on thethin film layer 8 and thin film type gas component detecting element 10of the present invention is formed to cover the electrode wires 9. Onthis detecting element 10 is formed a porous protective layer 11 oftitanium dioxide or aluminum oxide.

Substrate 7 having detecting element 10 is inserted into central hole 4aof small diameter insulator 4 made of aluminum oxide and fixed housing 2and is supported by collar 7b at an open end of central hole 4b oflarger diameter of insulator 4. This supporting portion is sealed byadhesive 5 filled in central hole 4b. Insulator tube 12 made of aluminumoxide is placed above adhesive 5. The above-mentioned heater wire 6 andelectrode wire 9 are electrically connected to lead wire 14 throughmetallic wire 13. To the lower end of housing 2 is welded a cylindricalmetallic protective tube having many openings 15a so that it surroundsdetecting element 10. In FIG. 13, 16 indicates a cylindrical support ofaluminum oxide, 17 indicates a spring, 18 indicates a sealer comprisingelectrically insulating ceramic, 19 indicates a cylindrical metallicprotective tube, 20 indicates a packing, and 21 indicates a mountingflange for fixing housing 2 to exhaust tube.

The production method of the present invention will be explained.

EXAMPLE 1

Particles of titanium dioxide having a specific surface area of 3.1 m²/g (first class grade chemical, rutile crystal) were calcined at about1000° C. for 1 hour to obtain particles of titanium dioxide having aspecific surface area of 1.5 m² /g. Then, the calcined particles oftitanium dioxide were charged in a pot made of aluminum oxide togetherwith balls of aluminum oxide and were ground for 10 hours to obtainparticles of titanium dioxide having a specific surface area of 3.0 m²/g.

The above ground particles of titanium dioxide were impregnated with 5wt. % (based on the weight of titanium dioxide) of a mixed solution ofchloroplatinic acid and rhodium chloride prepared so that molar fractionof platinum:rhodium was 9:1, then dried at 100° C. and heat treated at400° C. for 1 hour.

10 wt. % of ethyl cellulose binder was added to the particles oftitanium dioxide which supported the catalyst comprising the mixture ofplatinum-rhodium to obtain a paste. This paste as a gas componentdetecting element was coated at a thickness of about 150 μm on asubstrate as shown in FIG. 14(a) and (b) and sintered at about 1200° C.for 1 hour.

TEM (transmission electron microscope) image of the detecting elementpart of the sensor provided with this gas component detecting element isshown in FIG. 1. In FIG. 1, the narrow rectangular image at the centershows particles of titanium dioxide and the small circular image deep inshadow shows catalyst particles. As can be understood from FIG. 1, thereexisted catalyst particles of more than about 100 μm which had finishedgrowing due to the heat generated by sintering under high temperature ofabout 1200° C. for 1 hour and other particles which can grow were allwithin the above range of size.

On the other hand, according to the image of high magnification shown inFIG. 2, catalyst particles of several μm (fine particles) were nearlyuniformly deposited on the surface of particles of titanium dioxide.

Next, this sensor was subjected to endurance test under the followingconditions.

Endurance Test Condictions

Engine used--4 cylinders, 4 cycles, 3,000 cc

Temperature of detecting element part of sensor--about 900° C.

Endurance test time--100 hours

Results of the above endurance tests are shown in FIGS. 3 and 4. In FIG.3, circular images of deep shadow indicate catalyst particles and imagesof larger size than the circular images indicate particles of titaniumdioxide. In FIG. 4, images in the form of fine particles indicatecatalyst particles. As can be seen from these FIGS. 3 and 4, they showsubstantially no change form the initial state of FIGS. 1 and 2.

Next, endurance test according to emission evaluation of exhaust gas wasconducted under the following conditions.

Evaluation Conditions

Engine used--4 cylinders, 4 cycles, 1,600 cc

Temperature of detecting element part of sensor--about 900° C.

The emission evaluation of exhaust gas is conducted as follows: As shownin FIG. 15, gas component in exhaust gas from engine is detected bysensor S and the signal is processed by electrical control unit U toemit feed back signal to fuel feed part of engine E, thereby to adjustair fuel ratio (A/F) at the fuel feed part of engine and concentrationsof hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxide (NOx)after passing through ternary catalyst C at the state of the adjustedair fuel ratio (A/F) are measured. Control point in electrical controlunit is a value of theoretical air fuel ratio λ=1.0.

The results are shown in FIG. 16. As can be seen from FIG. 16, no changeof emission was recognized in the sensor of this Example.

From the above results, it can be seen that according to this Example,the catalyst particles deposited onto the surface of particles oftitanium dioxide do not move even by the heat under high temperature andas a result, stable catalytic activity can be obtained.

EXAMPLE 2

Particles of titanium dioxide having a specific surface area of 3.1 m²/g (first class grade chemical, rutile crystal) were calcined at about1300° C. for 1 hour to obtain particles of titanium dioxide having aspecific surface area of 0.8 m² /g. Then, the calcined particles oftitanium dioxide were charged in a pot made of aluminum oxide togetherwith balls of aluminum oxide and were ground for 10 hours to convert thespecific surface area of the particles of titanium dioxide to 2.0 m² /g.

Then, the same catalyst as used in Example 1 was supported on theparticles of titanium dioxide in the same manner as in Example 1 and agas component detecting element was formed on a substrate as in FIG.14(a) and (b) in the same manner as in Example 1.

TEM (transmission electron microscope) image of the detecting elementpart of the sensor provided with the gas component detecting elementobtained in this Example 2 is shown in FIG. 5. In FIG. 5, images in theform of fine particles indicate catalyst particles. Furthermore, TEMimage of the detecting element part which had been subjected to the sameendurance test as in Example 1 is shown in FIG. 6, wherein images in theform of fine particles indicate the catalyst particles.

Furthermore, endurance test according to emission evaluation of exhaustgas was conducted under the same conditions as in Example 1. The resultsare shown in FIG. 16.

It can be seen from FIG. 5, FIG. 6, and FIG. 16 that the number ofcatalyst particles in the form of fine particles was somewhat smallerthan that in Example 1, but catalytic activity nearly equal to that inExample 1 was obtained.

EXAMPLE 3

A gas component detecting element was obtained in the same manner as inExample 1 except that amount of the catalyst supported was 2 wt. % or 8wt. % in place of 5 wt. %.

TEM image of detecting element part of a sensor having the gas componentdetecting element obtained above was examined to find that it was nearlythe same as that of FIG. 2 and the catalyst particles were fine.Furthermore, there was substantially no difference in the finenessdepending on amount of the catalyst supported.

Endurance test according to emission evaluation of exhaust gas wasconducted under the same conditions as in Example 1. The results areshown in FIG. 16.

It can be seen from FIG. 16 that there was obtained catalytic activitynearly equal to that obtained in Example 1 where amount of the catalystwas 5 wt. %. According to experiments by the inventors, when amount ofthe catalyst supported is more than 20 wt. %, sintering of particles oftitanium dioxide is hindered and besides cost increases and thus thesupporting amount is preferably not more than 20 wt. %.

EXAMPLE 4

A gas component detecting element was obtained in the same manner as inExample 1 except that 5:5 was employed in place of 9:1 as molar fractionof platinum:rhodium of mixed solution of chloroplatinic acid and rhodiumchloride to be supported as catalyst.

TEM image of detecting element part of a sensor having the gas componentdetecting element obtained in this Example 4 is shown in FIG. 7, whereinimages in the form of fine particles indicate the catalyst particles.

Endurance test according to emission evaluation of exhaust gas wasconducted under the same conditions as in Example 1. The results areshown in FIG. 16.

It can be seen from FIG. 7 that there was no difference in state offineness of catalyst particles depending on amount of catalystsupported. Furthermore, as can be seen from FIG. 16, catalytic activityobtained was nearly the same as that in Example 1.

COMPARATIVE EXAMPLE 1

Particles of titanium dioxide having a specific surface area of 3.1 m²/g (first class grade chemical, rutile crystal) were calcined at about1000° C. for 1 hour to obtain particles of titanium dioxide having aspecific surface area of 1.5 m² /g. Then, the calcined particles oftitanium dioxide were charged in a pot made of aluminum oxide togetherwith balls of aluminum oxide and were ground for 10 hours to convert thespecific surface area of particles of titanium dioxide to 3.0 m² /g.

10 wt. % of ethyl cellulose binder was added to the above particles oftitanium dioxide to obtain a paste. This paste as a gas componentdetecting element was coated at a thickness of about 150 μm on asubstrate as shown in FIG. 14(a) and (b) and sintered at about 1200° C.for 1 hour. Specific surface area of the thus sintered particles oftitanium dioxide was 0.9 m² /g.

The resulting titanium dioxide sintered body was impregnated with amixed solution of chloroplatinic acid and rhodium chloride prepared sothat molar fraction of platinum:rhodium was 9:1 in an amount of 5 wt. %based on the weight of the sintered body. Then, the sintered body wasdried at 100° C. and baked at 800° C. for 1 hour to support the catalyston the sintered body.

TEM image of the detecting element part of a sensor having the gascomponent detecting element is shown in FIG. 8. In FIG. 8, the images inthe form of fine particles of deep shadow indicate the catalystparticles.

This sensor was subjected to the same endurance test as in Example 1 andTEM image of the detecting element part which had been subjected to theendurance test is shown in FIG. 9 and image of higher magnification isshown in FIG. 10. In FIG. 9, images in the form of fine particles deepin shadow indicate the catalyst particles. In FIG. 10, the surface ofthe oblique line indicates the surface of particles of titanium dioxideand streaks positioned above the oblique line indicate mesh of microgridof TEM.

As can be seen from FIG. 9, catalyst particles in the form of fineparticles of several nm in FIG. 8 disappeared by growth of particles andgrew to particles of several ten nm-several hundreds nm. Furthermore, ascan be seen from FIG. 10, no fine particles than the grown particleswere observed in the high magnification image.

The same endurance test according to emission evaluation as in Example 1was conducted to find change in control point as shown in FIG. 16.

COMPARATIVE EXAMPLE 2

Particles of titanium dioxide having a specific surface area of 3.1 m²/g (first class grade chemical, rutile crystal) were calcined at about1300° C. for 1 hour to obtain particles of titanium dioxide having aspecific surface area of 0.8 m² /g.

The above particles of titanium dioxide were impregnated with 5 wt. %(based on the weight of titanium dioxide) of a mixed solution ofchloroplatinic acid and rhodium chloride prepared so that molar fractionof platinum:rhodium was 9:1, then dried at 100° C. and heat treated at400° C. for 1 hour.

10 wt. % of ethyl cellulose binder was added to the particles oftitanium dioxide which supported catalyst comprising a mixture ofplatinum-rhodium to obtain a paste. This paste as a gas componentdetecting element was coated at a thickness of about 150 μm on asubstrate as shown in FIG. 14 (a) and (b) and sintered at about 1200° C.for 1 hour.

TEM image of detecting element part of a sensor provided with this gascomponent detecting element is shown in FIG. 11. In FIG. 11, thecircular images deep in shadow in the right portion indicate catalystparticles. High magnification image is shown in FIG. 12.

As can be seen from FIG. 11 and FIG. 12, since no catalyst particles inthe form of fine particles were deposited and catalytic activity waslow, the same emission evaluation as in Example 1 was not able to beconducted. This is because surface defects on the particles of titaniumdioxide disappeared due to the heat treatment at high temperature ofabout 1300° C. and the particles of titanium dioxide were not subjectedto treatment to reproduce surface defects by grinding the particles asin Examples 1-4.

What is claimed is:
 1. A method for producing a gas component detectingelement for detectors which comprises a sintered body of particles of ametal oxide which changes in its electrical resistance depending onrelative atmosphere of combustible components in an exhaust gas,catalyst particles being supported on the surface of particles of themetal oxide, the method comprising the following steps:preparing of themetal oxide, calcining the particles of the metal oxide, subjecting thecalcined particles from said calcining step to a treatment for aenlarging a specific surface area of the calcined particles and toincrease surface defects thereof, supporting catalyst particles on thesurface of the particles subjected to the enlargement treatment in saidsubjecting step, and sintering the particles which support the catalystparticles.
 2. A method according to claim 1, wherein the treatment forenlarging specific surface area of particles is conducted by grindingthe calcined particles.
 3. A method according to claim 1, wherein thetreatment for enlarging specific surface area of particles is conductedby dissolving the surface of the calcined particles.
 4. A methodaccording to claim 2, wherein ratio of specific surface area A₁ of thecalcined particles and specific surface area A₂ of the particlessubjected to the treatment for enlarging specific surface area satisfiesthe relation of A₁ /A₂ <1.0.
 5. A method according to claim 1, whereinthe particles of metal oxide are titanium dioxide.
 6. A method accordingto claim 5, wherein the catalyst particles comprise a mixture ofplatinum-rhodium.
 7. A method for producing a gas component detectingelement for detectors which comprises a sintered body of particles oftitanium dioxide which changes in its electrical resistance depending onrelative atmosphere of combustible components in an exhaust gas,catalyst particles being supported on the surface of the particles oftitanium dioxide, the method comprising the following steps:preparing ofthe titanium dioxide, calcining the particles of the titanium dioxide,subjecting the calcined particles from said calcining step to atreatment for a enlarging a specific surface area A₁ of the calcinedparticles before subjected to grinding and specific surface area A₂ ofthe particles after subjected to grinding satisfies the relation A₁ A₂<1.0, and to thereby increase surface defects thereof, supportingcatalyst particles on the surface of the particles subjected to thegrinding treatment, and sintering the particles which support thecatalyst particles.
 8. A method according to claim 7, wherein thecatalyst particles comprise a mixture of platinum-rhodium.
 9. A methodfor producing a gas component detecting element for detectors comprisingan electrical insulating substrate and the gas component detectingelement which comprises a sintered body of particles of titanium dioxidewhich changes in its electrical resistance depending on relativeatmosphere of combustible components in an exhaust gas, catalystparticles being supported on the surface of the particles of titaniumdioxide, the method comprising the following steps:preparing theparticles of titanium dioxide, calcining the particles of titaniumdioxide, subjecting the calcined particles to a grinding treatment sothat ratio of specific surface area A₁ of the calcined particles beforesubjected to grinding and specific surface area A₂ of the particlesafter subjected to grinding satisfies the relation A₁ /A₂ <1.0,impregnating the particles of titanium dioxide subjected to the grindingtreatment with a mixed solution of chloroplatinic acid and rhodiumchloride, adding an organic binder to the particles of titanium dioxideimpregnated with the mixed solution to obtain a paste, providing thepasty particles of titanium dioxide on a electrical insulating substratein the form of a film at a given thickness, and sintering the particlesof titanium dioxide provided on the substrate at 800°-1300° C.